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Learn how to effectively merge two tables in PostgreSQL using multiple conditions, ensuring unique entries and choosing specific values. --- This video is based on the question https://stackoverflow.com/q/68083122/ asked by the user 'Shivam Kapoor' ( https://stackoverflow.com/u/1879109/ ) and on the answer https://stackoverflow.com/a/68083143/ provided by the user 'Gordon Linoff' ( https://stackoverflow.com/u/1144035/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions. Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: How to merge two tables in postgresql based on multiple conditions Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l... The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license. If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com. --- How to Merge Two Tables in PostgreSQL Based on Multiple Conditions When working with database management systems like PostgreSQL, it's common to encounter scenarios where you need to merge data from multiple tables. In this guide, we'll be taking a close look at a specific example of how to merge two tables—namely, tables A and B—based on multiple conditions. We'll also address some common mistakes and clarify the best approaches to achieve the desired results. The Problem In our example, we have the following two tables: Table A [[See Video to Reveal this Text or Code Snippet]] With inserted values: [[See Video to Reveal this Text or Code Snippet]] Table B [[See Video to Reveal this Text or Code Snippet]] With inserted values: [[See Video to Reveal this Text or Code Snippet]] The goal is to merge these two tables so that: Table A retains its entries, Entries from Table B are considered, provided that the personid and writers are unique, and The job information is taken from Table A rather than Table B. Desired Output from Table A after the merge: [[See Video to Reveal this Text or Code Snippet]] Issues with Previous Attempts A common misstep is to use a simple UNION statement, like this: [[See Video to Reveal this Text or Code Snippet]] This would produce: [[See Video to Reveal this Text or Code Snippet]] However, this does not fulfill our criteria. The Solution: Using SQL Queries Effectively To achieve the desired outcome, we can utilize the SQL SELECT statement cleverly combined with UNION ALL. Here’s the breakdown: Step 1: Select All Entries from Table A Start by selecting all desired entries from Table A: [[See Video to Reveal this Text or Code Snippet]] Step 2: Select Non-Matching Entries from Table B Next, we want to get the entries from Table B that do not match with those in Table A. We can achieve this by using a WHERE NOT EXISTS clause: [[See Video to Reveal this Text or Code Snippet]] Step 3: Combine the Results Using UNION ALL Finally, combine both queries using UNION ALL: [[See Video to Reveal this Text or Code Snippet]] Final Thoughts By executing the SQL query above, you should now achieve the desired result of merging the two tables correctly while maintaining the integrity of your entries according to the specified constraints. Remember, clarity in your conditionals and understanding of SQL statements is crucial—this will help you avoid common pitfalls when merging tables in PostgreSQL. Happy querying!